Relationship between saccadic eye movements and cortical activity as measured by fMRI: quantitative and qualitative aspects

We investigated the quantitative relationship between saccadic activity (as reflected in frequency of occurrence and amplitude of saccades) and blood oxygenation level dependent (BOLD) changes in the cerebral cortex using fun ctional magnetic resonance imaging (fMRI). Furthermore, we investigated qua ntitative changes in cortical activity associated with qualitative changes in the saccade task for comparable levels of saccadic activity. All experim ents required the simultaneous acquisition of eve movement and fMRI data. F or this purpose we used a new high-resolution limbus-tracking technique for recording eve movements in the magnetic resonance tomograph. In the first two experimental series we varied both frequency and amplitude of saccade s timuli (target jumps'). In the third series we varied task difficulty, subj ects performed either pro-saccades or anti-saccades. The brain volume inves tigated comprised the frontal and supplementary eve fields, parietal as wel l as striate cortex, and the motion sensitive area of the parieto-occipital cortex. All these regions showed saccade-related BOLD responses. The respo nses in these regions were highly correlated with saccade frequency, indica ting that repeated processing of saccades is integrated over time in the BO LD response. In contrast. there was no comparable BOLD change with variatio n of saccade amplitude. This finding speaks for a topological rather than a ctivity-dependent coding of saccade amplitudes in most cortical regions. In the experiments comparing pros anti-saccades we found higher BOLD activati on in the V "anti" task than in the "pro" task. A comparison of saccade par ameters revealed that saccade frequency and cumulative amplitude were compa rable between the two tasks, whereas reaction times were longer in the "ant i" task than the pro task. The latter finding is taken to indicate a more d emanding cortical processing in the "anti" task than the "pro" task, which could explain the observed difference in BOLD activation. We hold that a qu antitative analysis of saccade parameters (especially saccade frequency and latency) is important for the interpretation of the BOLD changes observed with visual stimuli in fMRI.